1. Field of the Invention
This invention relates to a chlorine-containing resin composition having superior thermal stability, and more specifically to a chlorine-containing resin composition having superior thermal stability and comprising a specific mixture of a polyhydric alcohol and an ester derivative of a polyhydric alcohol as a stabilizer.
2. Description of the Prior Art
Chlorine-containing resins are intrinsically unstable to heat, and, when heated during molding, for example, chlorine-containing resins decompose with hydrogen chloride being evolved and color developing. To prevent coloration by heat decomposition, the previous practice has been to mold these resins after a heat stabilizer has been added to these resins.
Heat stabilizers employed heretofore have the disadvantage that they contain a heavy metal such as lead which is toxic.
Combinations of calcium stearate, zinc stearate, organotin compounds, nitrogen-containing stabilizers such as .beta.-aminocrotonic acid esters, and polyhydroxy compounds such as pentaerythritol or dipentaerythritol have been used as stabilizers having reduced toxicity. However, when these polyhydroxy compounds are used, coloration occurs at an early stage. Moreover, the impact strength of molded products containing a polyhydroxy compound such as pentaerythritol or dipentaerythritol as a stabilizer component is, unfortunately, decreased. Pentaerythritol or dipentaerythritol is difficult to pulverize into a fine powder. Furthermore, since these compounds have a high melting point (pentaerythritol has a melting point of 210.degree. to 235.degree. C., and dipentaerythritol has a melting point of 215.degree. to 225.degree. C.), they do not melt as the molding temperature for a chlorine-containing resin, but rather remain as coarse particles in the molded product. For this reason, the impact strength of the molded product is reduced.
Further, Japanese Patent Publication No. 38767/1973 (i.e., Japanese Published Examined Patent Application No. 38767/1973) describes an ester compound of pentaerythritol with maleic acid as a thermal stabilizer for a chlorine-containing resin. But pentaerythritol maleate does not provide sufficient thermal stability and sufficient impact strength of the chlorine-containing resin is not maintained.
Japanese Patent Publication No. 25257/1976 (i.e., Japanese Published Examined Patent Application No. 25257/1976) describes, as a stabilizer for a chlorine-containing resin, a eutectic mixture of dipentaerythritol and/or bisphenol A with pentaerythritol and having a melting point of less than the molding temperature of the chlorine-containing resin. But the eutectic mixture of dipentaerythritol and pentaerythritol has a melting temperature of more than 180.degree. C. and, the eutectic mixture of dipentaerythritol, bisphenol A and pentaerythritol also has a melting temperature of more than 180.degree. C. Therefore, the eutectic mixtures remain as coarse particles in the molded chlorine-containing resin after molding. Thus, the eutectic mixture does not provide sufficient thermal stability and sufficient impact strength of the chlorine-containing resin is not maintained.